Cleaners, what’s something you notice in houses that causes health problems for owners?

I've been cleaning houses for about a year, and I've noticed that kids get sick often in houses with "rubber duckie-type" bath toys. These toys get water inside and grow black mold. They cannot be cleaned effectively. Kids are often sick in these houses. I recommend to parents to get rid of this type of toy.

Curious if there are other hazards to health you have suspicions about in the houses you have cleaned?